In the first video coding standards and recommendations (up to MPEG-4 and H.264), the video was assumed to be rectangular and to be described in terms of a luminance channel and two chrominance channels. With MPEG-4, an additional channel carrying shape information has been introduced. Two modes are available to compress those channels: the INTRA mode, according to which each channel is encoded by exploiting the spatial redundancy of the pixels in a given channel of a single image, and the INTER mode, that exploits the temporal redundancy between separate images. The INTER mode relies on a motion-compensation technique, which describes an image from one or several image(s) previously decoded by encoding the motion of pixels from one image to the other. Usually, the image to be encoded is partitioned into independent blocks or macroblocks, each of them being assigned a motion vector. A prediction of the image is then constructed by displacing pixel blocks from the reference image(s) according to the set of motion vectors (luminance and chrominance channels share the same motion description). Finally, the difference (called the residual signal) between the image to be encoded and its motion-compensated prediction is encoded in the INTER mode to further refine the decoded image. However, the fact that all pixel channels are described by the same motion information is a limitation damaging the compression efficiency of the video coding system.